I've recently been replaying Omega Ruby and I enjoyed it quite a bit, easily the best post-gen 4 game simply because Hoenn is better than any region that has come since. It's not perfect and it could have been even better if it took more cues from Emerald (not just talking about the Battle Frontier, but the improved trainer teams and the option to choose between double and single battles for certain trainers). Although there are a few really stupid changes they made as well.
>Granite Cave requiring the Mach Bike to reach the lower levels, so certain Pokemon like Aron, Mawile/Sableye and Nosepass are completely unobtainable unless you backtrack later. And they didn't bother increasing the levels of the Pokemon on lower levels so if you do decide to backtrack to catch them, those Pokemon will be very underlevelled.
>New Mauville being replaced with pretty much nothing, which is a shame because they even gave Scorched Slab something extra
>Forcing you to teleport to Mauville after beating Norman. This was a great moment in the originals since you c9uld use your newly acquired Surf HM to either take the shortcut above Oldale, take the scenic route via Dewford and battle more trainers, or even surf north of Rustboro and find the island with Jigglypuff. The new change takes your incentive to explore these areas away from you. Plus they made other teleports optional (like the one after Flannery), so I don't see why they had to force this one.
>Giving you a stupidly powerful legendary with a Mega evolution for free was silly. Especially since you don't even unlock soaring until after beating Kyogre/Groudon, so there's no reason why they had to give it to you that early. You can just box it immediately and never use it, but it's forced into your party so you have to travel back to a Pokemon Center to do that.
